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Commands And Arguments

BASH reads commands from its input (which is either a terminal or a file). These commands can be aliases, functions, builtins, keywords, or executables.

Each command can be followed by arguments. Arguments are words you specify after the command name. Arguments are separated from the command name and from each other by white space. This is important to remember. For example, the following is wrong:

    $ [-f file]

You want the [ command name to be separated from the arguments -f, file and ]. If you do not separate [ and -f from each other with whitespace, bash will think you are trying to execute the command name [-f and look in PATH for a program named [-f. Additionally, the arguments file and ] also need to be separated by spaces. The [ command expects the last argument to be ]. The correct command separates all arguments with spaces:

    $ [ -f file ]

NOTE:
It is very important that you understand how this works exactly. If you don't grasp these concepts well, the quality of your code will degrade significantly and you will introduce very dangerous bugs. Read Argument Splitting very carefully.

    $ ls
    a  b  c

ls is a command that lists files in the current directory. It's intended to be used only for producing human-readable results. Please don't try to parse, pipe, grep, capture, read, or loop over the output of ls in a script. It's dangerous and there's always a better way. While an invaluable tool on the interactive shell, ls should never be used in scripts. You will understand why as you go through this guide.

    $ mkdir d
    $ cd d
    $ ls

mkdir is a command that creates a new directory. We specified the argument d to that command. This way, the application mkdir is instructed to create a directory called d. After that, we use the builtin command cd to change the shell's current directory to d. ls shows us that the current directory (which is now d) is empty, since it doesn't display any filenames.

In BASH scripts and functions, arguments that were passed to the script or function are saved in 'Positional Parameters'. You can read these by using $1, $2, and so on for the respective argument. You can also use $@ and $* but more about this later on.


    $ type rm
    rm is hashed (/bin/rm)
    $ type cd
    cd is a shell builtin




Argument Splitting

Commands in BASH can take multiple arguments. These arguments are used to tell the command exactly what it's supposed to do. In BASH, you separate these arguments by whitespace (spaces and tabs).

Assume you're in an empty directory. (If you want to try this code out, you can create and go into an empty directory called test by running: mkdir test; cd test.)

    $ ls                # List files in the current directory (no output: no files).
    $ touch a b c       # Create files 'a', 'b' and 'c'.
    $ ls                # List all files again; this time the output shows 'a', 'b' and 'c'.
    a  b  c

touch is an application that changes the 'Last Modified'-time of a certain file to the current time. If the filename that it's given does not exist yet, it simply creates that file, as a new and empty file. In this example, we passed three arguments. touch creates a file for each argument. ls shows us that three files have been created.

    $ rm *              # Remove all files in the current directory.
    $ ls                # List files in the current directory (no output: no files).
    $ touch a   b c     # Create files 'a', 'b' and 'c'.
    $ ls                # List all files again; this time the output shows 'a', 'b' and 'c'.
    a  b  c

rm is an application that removes all the files that it was given. * is a glob. It basically means all files in the current directory. You will read more about this later on.

Now, did you notice that there are several spaces between a and b, and only one between b and c? Also, notice that the files that were created by touch are no different than the first time. You now know that the amount of whitespace between arguments does not matter. This is important to know. For example:

    $ echo This is a test.
    This is a test.
    $ echo This    is    a    test.
    This is a test.

In this case, we provide the echo command with four arguments. 'This', 'is', 'a' and 'test.'. echo takes these arguments, and prints them out one by one with a space in between. In the second case, the exact same thing happens. The extra spaces make no difference. If we actually want the extra whitespace, we need to pass the sentence as one single argument. We can do this by using quotes:

    $ echo "This    is    a    test."
    This    is    a    test.

Quotes group everything inside them into a single argument. This argument is 'This    is    a    test.', properly spaced. Note that the quotes are not part of the argument; BASH removes them before handing the argument to echo. echo prints this single argument out just like it always does.

Be very careful to avoid the following:

    $ ls                                          # List files in the current directory.
    The secret voice in your head.mp3  secret
    $ rm The secret voice in your head.mp3        # Executes rm with 6 arguments; not 1!
    rm: cannot remove `The': No such file or directory
    rm: cannot remove `voice': No such file or directory
    rm: cannot remove `in': No such file or directory
    rm: cannot remove `your': No such file or directory
    rm: cannot remove `head.mp3': No such file or directory
    $ ls                                          # List files in the current directory: It is still there.
    The secret voice in your head.mp3

You need to make sure you quote filenames properly. If you don't you'll end up deleting the wrong things! rm takes filenames as arguments. If you do not quote filenames with spaces, rm thinks that each argument is a separate file. Since BASH splits your arguments at the spaces, rm will try to remove each word. The above example tried to delete files for each word in the filename of the song, instead of the filename of the song. That caused our file secret to be deleted, and our song to remain behind!

Please have a good look at Quotes, WordSplitting and http://bash-hackers.org/wiki/doku.php?id=syntax:words if all this isn't very clear to you yet.





Scripts

A script is basically a sequence of commands that BASH processes in order. It only moves on to the next command when the current one has ended, unless the current one has been executed asynchronously (in the background). Don't worry too much about the latter case yet -- you'll learn about how that works later on.

Virtually any example that you see in this guide can be used in a script just as well as on the command line.

Making a script is easy. You just make a new file, and put this in it at the top:

    #!/usr/bin/env bash

This header makes sure that whenever your script is executed, BASH will be used as its interpreter. Please do not be fooled by examples on the Internet that use /bin/sh as interpreter. sh is not bash. Even though sh's syntax and bash's look very much alike and even though most bash scripts will run in sh, a lot of the examples in this guide only apply to bash and will just break or cause unexpected behaviour in sh. Also, please refrain from giving your scripts that stupid .sh extension. It serves no purpose, and it's completely misleading (since it's going to be a bash script, not an sh script).

And by the way, it's perfectly fine if you use Windows to write your scripts, but if at all possible, avoid using Notepad for writing scripts. Microsoft Notepad can only make files with DOS-style line-endings. That means that each line you make in notepad will be ended by two characters: a Carriage Return and a Newline character. BASH reads lines as terminated by Newline characters only. As a result, the Carriage Return character will cause you incredible headache if you don't know it's there (very weird error messages). If at all possible, use a decent editor like Vim, Emacs, kate, GEdit, GVIM or xemacs. If you don't, then you will need to remove the carriage returns from your scripts before running them.

Once your script file has been made, you can run it like this:

    $ bash myscript

In this example, we execute BASH and tell it to read our script. Alternatively, you can give your script executable permissions. When you do this, you can actually execute the script instead of calling BASH manually:

    $ chmod +x myscript
    $ ./myscript

When executed in this way, the #! line tells the operating system what interpreter to use.

Some people like to keep their scripts in a personal directory. Others like to keep their scripts somewhere in the PATH variable. Most like to do both at once. Here's what I suggest you do:

    $ mkdir -p "$HOME/bin"
    $ echo 'PATH="$HOME/bin:$PATH"' >> "$HOME/.bashrc"

The first command will make a directory called bin in your home directory. The second command will add a line to your .bashrc file which adds the directory we just made to the beginning of the PATH variable. Every new instance of BASH will now check for executable scripts in your bin directory.

To apply the changes we added to .bashrc we obviously need to actually process .bashrc first. You can do that by closing your existing terminal and opening a new one. BASH will then initialize itself again by reading .bashrc (and possibly other files). Alternatively you can just execute that line of code on the command line (PATH="$HOME/bin:$PATH") or manually process your .bashrc file in the running shell by running source "$HOME/.bashrc" . Yet another way would be to replace your current BASH instance with a new one by running exec bash .

As a result, we can now put our script in our bin directory and execute it as a normal command (we no longer need to prepend our script's name with its path, which was the ./ part in the previous examples):

    $ mv myscript "$HOME/bin"
    $ myscript


    #! /usr/bin/env bash
    #
    #   scriptname argument [argument] ...
    #
    # A short explanation of your script's purpose.
    #
    # Copyright [date], [name]


    #! /bin/bash -xv

    #! /usr/bin/env bash -xv



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